SABnzbd only accepts NZB files as download specifications1.
An NZB file contains a full specification of all Usenet articles that are needed for a download. You can get these files from sites listed below.

SABnzbd does not endorse or require the use of any specific indexer.
They all work with SABnzbd perfectly fine so long as they generate NZBs, none of these sites require specific "SABnzbd integration".
Before paying for an indexer please keep in mind that there are no guarantees and that any site could disappear at any time without notice.

1. Usenet servers hold all kinds of material, sometimes of questionable origin and legality.
Please be careful and check if your downloading is legal in your country.
This is your responsibility, not that of the authors of SABnzbd or the sites offering the NZB files.

2. Most indexing sites do title filtering, where an NZB "release" (category data, etc) is constructed using filtering logic. Raw search indexers provide the group and raw header title for release names, which can be messy, but return more results.

3. Indexer tags allow sabnzbd to automatically assign the correct categories. Cat is the category of the release (i.e. TV, Movies, etc.) and Subcat is the subcategory (XviD, x264, etc).

4. The RSS URL for NZB.su needs authentication data.
First you need to register with the site.
Then follow these instructions.
Example: http://nzb.su/rss?t=-2&dl=1&i=<uid>&r=<hash>
Both the i and r values are required and account-specific.